Chlamydia is one of the most common sexually transmitted diseases, infecting around 1.4 million people in the United States in 2012. The infection is gnarly: it can lead to ectopic pregnancies, pneumonia, and infertility in women, and can also cause blindness if you get the infection in your eyes (which you should really try not to do). Despite the severe side effects, scientists have never been able to develop a successful preventive vaccine—until, quite possibly, now.

The mission to create a chlamydia vaccine had previously been largely abandoned after scientists tested inoculations on healthy people in countries like Saudi Arabia, India and Ethiopia in the 1960s. Not only were these trials outrageously unethical, but the vaccines just didn’t really work—and, in some cases, made people more likely to contract the disease.

In a new study published in the journal Science, a team of researchers basically recreated these failed experiments in mice, injecting them with either live or dead chlamydia bacteria, and then a second dose of live chlamydia to see how the animals would respond.

The Verge’s Arielle Duhaime-Ross explains:

A specific type of white blood cell, called a T lymphocyte, was responsive to Chlamydia, an immune system analysis showed. Live and dead bacteria elicited different immune responses from the T lymphocytes. T cells can become cells that fight the infection — protective T cells — or regulatory T cells, which protect Chlamydia. [...] Dead Chlamydia exposure created regulatory T cells, making the infection more effective in the mice.

In other words, dead chlamydia suppressed the little mouse immune system, allowing the disease to spread way more easily.

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So, in order to effectively brainwash the mice’s T cells into thinking that chlamydia should be destroyed, the researchers incorporated a nanoparticle adjuvant into the vaccine, which was ultimately successful in boosting the cells’ immune response. Since chlamydia mainly infects the uterus (which is covered in mucosa, not skin), the researchers opted to deliver the inoculation via a nasal spray to more effectively target that type of tissue. The formula resulted in an average six months of protection for the mice.

Even though the vaccine won’t be available for human testing for several years (science is slow), the fact that researchers are even working on preventing STIs—today, vaccines only exist for HPV and hepatitis A and B—is a major win for reproductive and sexual health.

Image via Getty

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